Transparent inteconnection of ethernet fabric switches

ABSTRACT

One embodiment of the present invention provides a switch. The switch includes a fabric switch module and a border module. The fabric switch module maintains a membership in a first fabric switch. The fabric switch includes a plurality of switches and operates as a single logical switch. The border module determines that the egress switch identifier in a first encapsulation header of a first packet is associated with a switch outside of the fabric switch. The first packet is forwardable in the first fabric switch based on the first encapsulation header. In response to the determination, the border module changes the ingress switch identifier in the first encapsulation header of the first packet to a first virtual switch identifier associated with a first virtual switch. This first virtual switch externally represents the first fabric switch.

RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 61/874,919, Attorney Docket Number BRCD-3228.0.US.PSP, titled “Transparent Inter Ethernet Fabric Switch Routing,” by inventors Venkata R. K. Addanki, Mythilikanth Raman, and Shunjia Yu, filed 6 Sep. 2013, the disclosure of which is incorporated by reference herein.

The present disclosure is related to U.S. patent application Ser. No. 13/087,239, Attorney Docket Number BRCD-3008.1.US.NP, titled “Virtual Cluster Switching,” by inventors Suresh Vobbilisetty and Dilip Chatwani, filed 14 Apr. 2011; and U.S. patent application Ser. No. 12/725,249, Attorney Docket Number BRCD-112-0439US, titled “Redundant Host Connection in a Routed Network,” by inventors Somesh Gupta, Anoop Ghanwani, Phanidhar Koganti, and Shunjia Yu, filed 16 Mar. 2010, the disclosures of which are incorporated by reference herein.

BACKGROUND

1. Field

The present disclosure relates to network design. More specifically, the present disclosure relates to a method for a constructing a scalable switching system.

2. Related Art

The exponential growth of the Internet has made it a popular delivery medium for a variety of applications running on physical and virtual devices. Such applications have brought with them an increasing demand for bandwidth. As a result, equipment vendors race to build larger and faster switches with versatile capabilities. However, the size of a switch cannot grow infinitely. It is limited by physical space, power consumption, and design complexity, to name a few factors. Furthermore, switches with higher capability are usually more complex and expensive. More importantly, because an overly large and complex system often does not provide economy of scale, simply increasing the size and capability of a switch may prove economically unviable due to the increased per-port cost.

A flexible way to improve the scalability of a switch system is to build a fabric switch. A fabric switch is a collection of individual member switches. These member switches form a single, logical switch that can have an arbitrary number of ports and an arbitrary topology. As demands grow, customers can adopt a “pay as you grow” approach to scale up the capacity of the fabric switch.

Meanwhile, layer-2 (e.g., Ethernet) switching technologies continue to evolve. More routing-like functionalities, which have traditionally been the characteristics of layer-3 (e.g., Internet Protocol or IP) networks, are migrating into layer-2. Notably, the recent development of the Transparent Interconnection of Lots of Links (TRILL) protocol allows Ethernet switches to function more like routing devices. TRILL overcomes the inherent inefficiency of the conventional spanning tree protocol, which forces layer-2 switches to be coupled in a logical spanning-tree topology to avoid looping. TRILL allows routing bridges (RBridges) to be coupled in an arbitrary topology without the risk of looping by implementing routing functions in switches and including a hop count in the TRILL header.

While a fabric switch brings many desirable features to a network, some issues remain unsolved in efficiently interconnecting a plurality of fabric switches.

SUMMARY

One embodiment of the present invention provides a switch. The switch includes a fabric switch module and a border module. The fabric switch module maintains a membership in a first fabric switch. The fabric switch includes a plurality of switches and operates as a single logical switch. The border module determines that the egress switch identifier in a first encapsulation header of a first packet is associated with a switch outside of the fabric switch. The first packet is forwardable in the first fabric switch based on the first encapsulation header. In response to the determination, the border module changes the ingress switch identifier in the first encapsulation header of the first packet to a first virtual switch identifier associated with a first virtual switch. This first virtual switch externally represents the first fabric switch.

In a variation on this embodiment, the egress switch identifier in the first encapsulation header is a second virtual switch identifier associated with a second virtual switch, which externally represents a second fabric switch.

In a further variation, routing information of the first fabric switch indicates that the second virtual switch is reachable via the switch.

In a further variation, the border module determines that the egress switch identifier in a second encapsulation header of a second packet is the first virtual switch identifier. In response to the determination, the border module changes the egress switch identifier in the second encapsulation header of the second packet to a switch identifier which identifies a member switch in the first fabric switch.

In a further variation, the ingress switch identifier in the second encapsulation header of the second packet is the second virtual switch identifier.

In a further variation, the switch also includes a forwarding module which determines that the egress switch identifier in a third encapsulation header of a third packet is a switch identifier of the switch. The ingress switch identifier in the second encapsulation header of the third packet is the second virtual switch identifier. The switch also includes a learning module which learns a media access control (MAC) address from an inner packet of the third packet and stores the learned MAC address in association with the second virtual switch identifier in a storage device.

In a further variation on this embodiment, the switch also includes a forwarding module which determines an external switch as a next-hop switch for the first packet based on the first encapsulation header. This external switch is not a member switch of the first fabric switch.

In a further variation on this embodiment, the first encapsulation header is one or more of: (i) a Transparent Interconnection of Lots of Links (TRILL) header, wherein the ingress and egress switch identifiers of the first encapsulation header are TRILL routing bridge (RBridge) identifiers; and (ii) a Internet Protocol (IP) header, wherein the ingress and egress switch identifiers of the first encapsulation header are IP addresses.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates exemplary transparent interconnections of fabric switches, in accordance with an embodiment of the present invention.

FIG. 2A illustrates an exemplary forwarding of a packet with an unknown destination between transparently interconnected fabric switches, in accordance with an embodiment of the present invention.

FIG. 2B illustrates an exemplary forwarding of a packet with a known destination between transparently interconnected fabric switches, in accordance with an embodiment of the present invention.

FIG. 3A presents a flowchart illustrating the process of an edge switch forwarding a packet with an unknown destination received from a local end device, in accordance with an embodiment of the present invention.

FIG. 3B presents a flowchart illustrating the process of an egress border switch forwarding a packet with an unknown destination, in accordance with an embodiment of the present invention.

FIG. 3C presents a flowchart illustrating the process of an ingress border switch forwarding a packet with an unknown destination, in accordance with an embodiment of the present invention.

FIG. 4A presents a flowchart illustrating the process of an edge switch forwarding a packet with a known destination received from a local end device, in accordance with an embodiment of the present invention.

FIG. 4B presents a flowchart illustrating the process of an egress border switch forwarding a packet with a known destination, in accordance with an embodiment of the present invention.

FIG. 4C presents a flowchart illustrating the process of an ingress border switch forwarding a packet with a known destination, in accordance with an embodiment of the present invention.

FIG. 5 illustrates an exemplary switch with transparent fabric switch interconnection support, in accordance with an embodiment of the present invention.

In the figures, like reference numerals refer to the same figure elements.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present invention. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the claims.

Overview

In embodiments of the present invention, the problem of efficiently coupling a plurality of fabric switches is solved by representing a fabric switch as a single switch to another other fabric switch. For example, the single switch can be a virtual switch. Member switches of this other fabric switch view the virtual switch as another switch with fabric encapsulation support and forward a packet to the virtual switch based on the encapsulation header of the packet. In this way, the packet is transparently forwarded between two fabric switches.

With existing technologies, the member switches of a fabric switch are associated with the same fabric identifier of the fabric switch. When a new member switch joins the fabric switch, the fabric identifier becomes associated with the new member switch. Once a fabric switch is formed, its forwarding information (e.g., the learned media access control (MAC) addresses and corresponding virtual local area network (VLAN) tags) is shared among its member switches. However, when the number of member switches in the fabric switch increases, the performance of the fabric switch may deteriorate. For example, a respective member switch maintains configuration data and forwarding information of a respective other member switch. As the number of member switches becomes large, managing such information can require significant hardware and/or software resources, leading to deterioration of the performance of the fabric switch.

On the other hand, instead of a large fabric switch, the switches can form a plurality of interconnected fabric switches. As a result, few member switches (can be referred to as border switches) participate in a plurality of fabric switches, leading to additional hardware and management constraints on those border switches. For example, to ensure proper traffic isolation, a border switch can be aware of the VLANs of a respective fabric switch. Furthermore, when this border switch forwards traffic across different fabric switches, the border switch learns MAC addresses of end devices coupled to different fabric switches. Moreover, these border switches may need to participate in multiple instances of routing protocols in different fabric switches. As a result, interconnecting a plurality of fabric switches may not scale well.

To solve this problem, a fabric switch is represented as a virtual switch to its neighbor fabric switches. For example, a fabric switch externally appears as a single virtual switch, which supports the fabric encapsulation, to other fabric switches and/or compliant networks. This allows the other fabric switches and/or compliant networks to forward a packet to that fabric switch based on the encapsulation header of that packet without learning forwarding and configuration information of individual switches. The border member switches of the fabric switch, which adjoin the other fabric switches and/or compliant networks, can translate between the switch identifiers of virtual and physical switches, and perform the corresponding route lookup for corresponding network. As a result, a large number of switches can form a large network, which is isolated into small manageable fabric switches.

Upon receiving a packet from an adjoining fabric switch and/or compliant network, a border member switch decides how to forward that packet within the local fabric switch. Furthermore, when a member switch of the local fabric switch learns MAC addresses of end devices coupled to a remote fabric switch, the member switch stores the learned MAC address with the virtual switch representing that remote fabric switch instead of the individual member switch to which that end device is coupled. As a result, the member switch does not need to maintain configuration and routing information of individual member switches of the remote fabric switch. It should be noted that a remote fabric switch is a fabric switch in which a member switch of a local fabric switch does not participate and whose fabric identifier is not associated with the member switch.

In a fabric switch, any number of switches coupled in an arbitrary topology may logically operate as a single switch. The fabric switch can be an Ethernet fabric switch or a virtual cluster switch (VCS), which can operate as a single Ethernet switch. Any member switch may join or leave the fabric switch in “plug-and-play” mode without any manual configuration. In some embodiments, a respective switch in the fabric switch is a Transparent Interconnection of Lots of Links (TRILL) routing bridge (RBridge). In some further embodiments, a respective switch in the fabric switch is an Internet Protocol (IP) routing-capable switch (e.g., an IP router). The TRILL protocol is described in Internet Engineering Task Force (IETF) Request for Comments (RFC) 6325, titled “Routing Bridges (RBridges): Base Protocol Specification,” available at http://datatracker.ietf.org/doc/rfc6325/, which is incorporated by reference herein.

It should be noted that a fabric switch is not the same as conventional switch stacking. In switch stacking, multiple switches are interconnected at a common location (often within the same rack), based on a particular topology, and manually configured in a particular way. These stacked switches typically share a common address, e.g., an IP address, so they can be addressed as a single switch externally. Furthermore, switch stacking requires a significant amount of manual configuration of the ports and inter-switch links. The need for manual configuration prohibits switch stacking from being a viable option in building a large-scale switching system. The topology restriction imposed by switch stacking also limits the number of switches that can be stacked. This is because it is very difficult, if not impossible, to design a stack topology that allows the overall switch bandwidth to scale adequately with the number of switch units.

In contrast, a fabric switch can include an arbitrary number of switches with individual addresses, can be based on an arbitrary topology, and does not require extensive manual configuration. The switches can reside in the same location, or be distributed over different locations. These features overcome the inherent limitations of switch stacking and make it possible to build a large “switch farm,” which can be treated as a single, logical switch. Due to the automatic configuration capabilities of the fabric switch, an individual physical switch can dynamically join or leave the fabric switch without disrupting services to the rest of the network.

Furthermore, the automatic and dynamic configurability of the fabric switch allows a network operator to build its switching system in a distributed and “pay-as-you-grow” fashion without sacrificing scalability. The fabric switch's ability to respond to changing network conditions makes it an ideal solution in a virtual computing environment, where network loads often change with time.

In this disclosure, the term “fabric switch” refers to a number of interconnected physical switches which form a single, scalable logical switch. These physical switches are referred to as member switches of the fabric switch. In a fabric switch, any number of switches can be connected in an arbitrary topology, and the entire group of switches functions together as one single, logical switch. This feature makes it possible to use many smaller, inexpensive switches to construct a large fabric switch, which can be viewed as a single logical switch externally. Although the present disclosure is presented using examples based on a fabric switch, embodiments of the present invention are not limited to a fabric switch. Embodiments of the present invention are relevant to any computing device that includes a plurality of devices operating as a single device.

The term “end device” can refer to any device external to a fabric switch. Examples of an end device include, but are not limited to, a host machine, a conventional layer-2 switch, a layer-3 router, or any other type of network device. Additionally, an end device can be coupled to other switches or hosts further away from a layer-2 or layer-3 network. An end device can also be an aggregation point for a number of network devices to enter the fabric switch. An end device can also host one or more virtual machines.

The term “switch” is used in a generic sense, and it can refer to any standalone or fabric switch operating in any network layer. “Switch” should not be interpreted as limiting embodiments of the present invention to layer-2 networks. Any device that can forward traffic to an external device or another switch can be referred to as a “switch.” Any physical or virtual device (e.g., a virtual machine/switch operating on a computing device) that can forward traffic to an end device can be referred to as a “switch.” Examples of a “switch” include, but are not limited to, a layer-2 switch, a layer-3 router, a TRILL RBridge, or a fabric switch comprising a plurality of similar or heterogeneous smaller physical and/or virtual switches.

The term “edge port” refers to a port on a fabric switch which exchanges data frames with a network device outside of the fabric switch (i.e., an edge port is not used for exchanging data frames with another member switch of a fabric switch). The term “inter-switch port” refers to a port which sends/receives data frames among member switches of a fabric switch. If a switch is not a member of the local fabric switch and is capable of forwarding based on the encapsulation header of the fabric encapsulation, the inter-switch port coupling this switch can be referred to as a “border inter-switch port.” The terms “interface” and “port” are used interchangeably.

The term “switch identifier” refers to a group of bits that can be used to identify a switch. Examples of a switch identifier include, but are not limited to, a media access control (MAC) address, an Internet Protocol (IP) address, and an RBridge identifier. Note that the TRILL standard uses “RBridge ID” (RBridge identifier) to denote a 48-bit intermediate-system-to-intermediate-system (IS-IS) System ID assigned to an RBridge, and “RBridge nickname” to denote a 16-bit value that serves as an abbreviation for the “RBridge ID.” In this disclosure, “switch identifier” is used as a generic term, is not limited to any bit format, and can refer to any format that can identify a switch. The term “RBridge identifier” is also used in a generic sense, is not limited to any bit format, and can refer to “RBridge ID,” “RBridge nickname,” or any other format that can identify an RBridge.

The term “packet” refers to a group of bits that can be transported together across a network. “Packet” should not be interpreted as limiting embodiments of the present invention to layer-3 networks. “Packet” can be replaced by other terminologies referring to a group of bits, such as “message,” “frame,” “cell,” or “datagram.” The terms “packet” and “frame” are used interchangeably.

Network Architecture

FIG. 1 illustrates exemplary transparent interconnections of fabric switches, in accordance with an embodiment of the present invention. As illustrated in FIG. 1, a network 100 includes fabric switches 102, 103, and 104. These fabric switches are interconnected via a network 101. In some embodiments, network 101 is a fabric switch as well (under such circumstances, network 101 is also referred to as fabric switch 101). Fabric switch 102 includes member switches 122, 124, and 126; fabric switch 103 includes member switches 132, 134, 136, and 138; and fabric switch 104 includes member switches 142, 144, and 146. Network 101 includes switches 112, 114, 116, and 118. It should be noted that if network 101 is a fabric switch, switches 112, 114, 116, and 118 operate as member switches of fabric switch 101. End device 162 is coupled to fabric switch 104 via switch 144 and end device 164 is coupled to fabric switch 102 via switch 122. A member switch, such as switches 144 or 122, which couples an end device via an edge port can be referred to as an edge switch.

In some embodiments, fabric switches 102, 103, and 104 internally operate as respective TRILL networks (e.g., forward data packet based on the TRILL protocol). Then network 101 can be a compatible TRILL network, or a fabric switch which internally operates as a TRILL network. A respective member switch of network 101 and fabric switches 102, 103, and 104 can then be a TRILL RBridge (e.g., has an RBridge identifier which identifies a member switch in the corresponding fabric switch). In some further embodiments, fabric switches 102, 103, and 104 internally operate as respective IP networks (e.g., forward data packet based on the IP protocol). Then network 101 can be a compatible IP network, or a fabric switch which internally operates as an IP network. A respective member switch of network 101 and fabric switches 102, 103, and 104 can then be an IP-capable switch (e.g., has an IP address which identifies a member switch in the corresponding fabric switch and/or a larger network). An IP-capable switch can calculate and maintain a local IP routing table (e.g., a routing information base or RIB), and is capable of forwarding packets based on its IP addresses.

Switches in a fabric switch use edge ports to communicate with end devices (e.g., non-member switches) and inter-switch ports to communicate with other member switches. Data communication via an edge port can be based on Ethernet and via an inter-switch port can be based on IP and/or TRILL protocol. For example, switch 122 of fabric switch 102 is coupled to end device 164 via an edge port and to switches 124 and 128 via inter-switch ports and one or more links. Switch 122 can communicate with end device 164 based on Ethernet and with switch 124 and 126 based on IP or TRILL. It should be noted that control message exchange via inter-switch ports can be based on a different protocol (e.g., Internet Protocol (IP) or Fibre Channel (FC) protocol).

Furthermore, a switch in a fabric switch coupled with a switch in another fabric switch or a compatible network via a border inter-switch port. For example, switch 124 of fabric switch 102 is coupled with switch 112 of fabric switch 101 (or a compatible network 101) via a border inter-switch port. Forwarding to and/or via a compatible network does not require decapsulation of a fabric encapsulation. For example, if fabric encapsulation for fabric switch 102 is based on the TRILL protocol, switch 124 can forward a TRILL-encapsulated packet to and/or via network 101 (i.e., to switch 112) based on TRILL forwarding.

In some embodiments, fabric switch 102 is assigned a fabric switch identifier. A respective member switch of fabric switch 102 is associated with that fabric switch identifier. This allows a member switch to indicate that it is a member of fabric switch 102. In some embodiments, whenever a new member switch joins fabric switch 102, the fabric switch identifier is automatically associated with that new member switch. Similarly, fabric switch 103 and 104 (and fabric switch 101) are assigned corresponding fabric switch identifiers. Furthermore, a respective member switch of fabric switch 102 is assigned a switch identifier (e.g., an RBridge identifier, a Fibre Channel (FC) domain ID (identifier), or an IP address). This switch identifier identifies the member switch in fabric switch 102. Similarly, a respective member switch of fabric switch 103 and 104 (and fabric switch 101) is assigned a switch identifier.

With existing technologies, switches 122, 124, and 126 of fabric switch 102 are associated with the same fabric identifier of fabric switch 102. When a new member switch joins fabric switch 102, the fabric identifier becomes associated with that new member switch. Once fabric switch 102 is formed, its forwarding information (e.g., the learned MAC addresses and corresponding virtual local area network (VLAN) tags) is shared among member switches 122, 124, and 126. As a result, a respective member switch of fabric switch 102 maintains a large number of learned MAC address and its association with a member switch. As a result, if the number of member switches in fabric switch 102 increases, the performance of fabric switch 102 may deteriorate. For example, switch 122 maintains configuration data and forwarding information of switches 124 and 126. This allows switch 122 to forward packets to MAC address associated with switch 124 and 126. As the number of member switches in fabric switch 102 becomes large, managing such information can deteriorate the performance of fabric switch 102.

On the other hand, instead of a large fabric switch, the switches in network 100 can form a plurality of interconnected fabric switches 101, 102, 103, and 104. As a result, few member switches, which can be referred to as border switches, may participate in a plurality of fabric switches. For example, border switches 124 and 126 may participate in fabric switch 101 in addition to their local fabric switch 102. As a result, switch 124 and 126 maintain learned MAC addresses, forwarding information, and configuration information of both fabric switches 101 and 102. This leads to additional hardware and management constraints on switches 124 and 126.

Furthermore, to ensure proper traffic isolation, border switches 124 and 126 is aware of VLAN configurations of both fabric switches 101 and 102. For example, to ensure VLAN continuity, when forwarding a packet to switch 112 in fabric switch 101, switch 124 checks whether the VLAN of the packet is configured in switch 112. As a result, in addition to VLAN configurations of switches 122 and 126, switch 124 maintains VLAN configurations of switches 112, 114, 116, and 118. Similarly, border switches 112 and 114 of fabric switch 101 can be aware of VLAN configurations of both fabric switches 101 and 102. This can also lead to additional hardware and management constraints on these border switches.

Moreover, if border switch 112 receives traffic from end device 164 via switch 124, switch 112 learns MAC addresses of end device 164 and its association information with switch 122. Switch 112 shares this information with switches 114, 116, and 118. As a result, switches 112, 114, 116, and 118 maintain MAC address of end device 164 and its association with switch 122, as well as forwarding information for switch 122. To ensure packet forwarding between fabric switches 101 and 102, border switches 112, 114, 124, and 126 participate in respective instances of routing protocols of fabric switches 101 and 102. For example, border switch 112 computes route to switch 118 based on the routing protocol instance of fabric switch 101 and route to switch 122 based on the routing protocol instance of fabric switch 102. As a result, interconnecting a plurality of fabric switches may not scale well.

To solve this problem, fabric switch 102 is represented as a virtual switch 120 (denoted with dotted lines) to other fabric switches and compatible networks of network 100. For example, fabric switch 102 appears as virtual switch 120 to network 101, and fabric switches 103 and 104. Switches 112 and 114 consider themselves to be coupled to virtual switch 120. In other words, interconnections between switches 112 and 114 with switches 124 and 126 are represented to switches 112 and 114 as interconnections between switches 112 and 114 with virtual switch 120. Similarly, fabric switch 103 is represented as a virtual switch 130 (denoted with dotted lines) to other fabric switches and compatible networks of network 100. Fabric switch 103 appears as virtual switch 130 to network 101, and fabric switches 102 and 104. Switches 116 and 118 consider themselves to be coupled to virtual switch 130. In the same way, fabric switch 104 is represented as a virtual switch 140 (denoted with dotted lines) to other fabric switches and compatible networks of network 100. Fabric switch 104 appears as virtual switch 140 to network 101, and fabric switches 102 and 103. Switch 116 considers itself to be coupled to virtual switch 140. If network 101 operates as a fabric switch, network 101 is represented as a virtual member switch 110 (denoted with dashed lines) to other fabric switches of network 100.

Switches in fabric switch 104 consider virtual switch 120 to be reachable via switch 142. Routing information in fabric switch 104 indicates that virtual switch 120 is reachable via switch 142. Routing, forwarding, and failure recovery of a fabric switch is specified in U.S. patent application Ser. No. 13/087,239, Attorney Docket Number BRCD-3008.1.US.NP, titled “Virtual Cluster Switching,” by inventors Suresh Vobbilisetty and Dilip Chatwani, filed 14 Apr. 2011, the disclosure of which is incorporated herein in its entirety. Similarly, switches in fabric switch 103 consider virtual switch 120 to be reachable via switches 132 and 136, and switches in fabric switch 101 consider virtual switch 120 to be reachable via switches 112 and 114, and compatible network 101. For the packets from fabric switch 102, switches 124 and 126 can translate between the switch identifier of virtual switch 120 and a corresponding physical switch, and perform the route lookup. As a result, in the links between fabric switch 102 and compatible network 101, the ingress and/or egress switch identifiers of a packet can be virtual switch identifiers. This allows a large number of switches to form a large network 100, which is isolated into small manageable fabric switches 102, 103, and 104 interconnected via compatible network 101.

During operation, end device 164 sends packet to end device 162. This packet can be an Ethernet frame. Switch 122 receives the packet via an edge port and encapsulates the packet with a fabric encapsulation (e.g., TRILL or IP encapsulation) and forwards. If switch 122 does not know the MAC address of end device 162, switch 122 assigns an “all-switch” switch identifier as the egress switch identifier of the encapsulation header. An “all-switch” switch identifier indicates that the fabric-encapsulated packet should be forwarded to a respective switch in a network. For example, if switch 132 receives a packet with an “all-switch” switch identifier as the egress identifier, switch 132 forwards the packet to switches 134, 136, and 138 of fabric switch 103.

When the encapsulated packet from switch 122 reaches border switch 124 (or 126), switch 124 modifies the encapsulation header by changing the ingress switch identifier of the encapsulation header from the switch identifier (e.g., an RBridge identifier or an IP address) of switch 122 to a virtual switch identifier (e.g., a virtual RBridge identifier or a virtual IP address) of virtual switch 120. Switch 124 forwards that packet to switch 112. In this way, switch 124 operates as an egress border switch, which forwards a fabric-encapsulated packet to outside of the local fabric switch (e.g., fabric switch 104) via a border inter-switch port. It should be noted that forwarding includes determining an egress (or output) port associated with the destination address and transmitting via the determined egress port.

Upon receiving the fabric-encapsulated packet, switch 112 determines that the egress switch identifier is an “all-switch” switch identifier and forwards the fabric-encapsulated packet to a respective switch in network 101. Upon receiving the packet, switch 116 and 118 forward the packet to switch 142 of fabric switch 104 and switch 132 of fabric switch 103, respectively. It appears to switch 116 and 118 that the packet is forwarded to virtual switch 140 and 130, respectively. In this way, compliant network 101 can forward the packet based on the encapsulation header among fabric switches coupled to network 101. Upon receiving the fabric-encapsulated packet, switch 142 determines that the egress switch identifier is an “all-switch” switch identifier and forwards the fabric-encapsulated packet to a respective switch in fabric switch 104. In this way, switch 142 operates as an ingress border switch, which forwards a fabric-encapsulated packet received via a border inter-switch in the local fabric switch (e.g., fabric switch 104).

Switch 142 can also decapsulate the fabric-encapsulated packet to obtain the inner packet (i.e., the Ethernet packet from end device 164) and determine whether any local end device is the destination of the inner packet. Switch 142 can learn the MAC address of end device 164 and stores the learned MAC address in association with the virtual switch identifier of virtual switch 120. Similarly, switch 144 receives the fabric-encapsulated packet, decapsulates the packet to obtain the inner packet, and determines whether any local end device is the destination of the inner packet. Switch 144 learns the MAC address of end device 164 and stores the learned MAC address in association with the virtual switch identifier of virtual switch 120. Switch 144 also determines that destination end device 162 is locally coupled (e.g., either based on a populated table, previous MAC address learning, or flooding), and forwards the inner packet to end device 162. In this way, fabric switches 102, 103, and 104 are transparently interconnected via compatible network 101.

In some embodiments, switches in network 101 decapsulate the fabric-encapsulated packet to determine whether any local end device is the destination of the inner packet (i.e., the Ethernet packet from end device 164). If a switch in network 101 learns the MAC address of end device 164, that switch stores the learned MAC address in association with the ingress switch identifier, which is a virtual switch identifier, in the encapsulation header. For example, if network 101 is a TRILL network and switch 112 receives a TRILL-encapsulated packet from fabric switch 102, upon decapsulating the TRILL header, switch 112 can learn the MAC address of the inner packet and stores the learned MAC address in association with the virtual switch identifier of virtual switch 120.

The routes (e.g., can be configured or computed based on a routing protocol) in fabric switch 102 indicate that virtual switch 120 is reachable via switch 142. Hence, to send a packet to end device 164, a switch in fabric switch 104 forwards the packet to switch 142. For example, if end device 162 sends a packet to end device 164, switch 144 receives the packet, encapsulates the packet in a fabric encapsulation, and assigns the virtual switch identifier of virtual switch 120 as the egress switch identifier of the encapsulation header. When the encapsulated packet reaches border switch 142, switch 142 modifies the encapsulation header by changing the ingress switch identifier of the encapsulation header from the switch identifier of switch 144 to a virtual switch identifier of virtual switch 140. Switch 142 forwards that packet to switch 116. Switch 116 determines that virtual switch 120 is reachable via switches 112 and 114. Suppose that switch 116 forwards the packet to switch 112, which in turn, forwards the packet to switch 124. Switch 124 changes the egress switch identifier of the encapsulation header from the virtual switch identifier of virtual switch 120 to switch identifier of switch 122, and forwards the packet.

Hence, in network 100, border switches 124 and 126 of fabric switch 102 do not need to participate in routing instances of fabric switches 103 and 104, and maintain forwarding information for individual switches of fabric switches 103 and 104. Since switches 124 and 126 are at the edge between network 101 and fabric switch 102, switches 124 and 126 determines how to forward packets received from network 101 within fabric switch 102. Furthermore, when switch 124 or 126 learns the MAC address of an end device coupled to a remote fabric (e.g., end device 162 coupled to fabric switch 104), switch 124 or 126 associates the learned MAC address with the virtual switch identifier of fabric switch 104 rather than the switch identifier of switch 144.

In some embodiments, network 101 can be any network which allows forwarding of fabric encapsulated packets based on encapsulation headers. For example, if the fabric encapsulation is based on the TRILL protocol, switches 112, 114, 116, and 118 can forward a packet based on the ingress and egress TRILL RBridge identifiers in the TRILL header of a TRILL encapsulated packet. This allows fabric switches 102, 103, and 104 to interconnect via a compatible network 101 without requiring network 101 to be a fabric switch. As result, network 101 can provide interconnection among fabric switches 102, 103, and 104 without providing connectivity within a fabric switch. Border switches (e.g., switches 124 and 126 of fabric switch 102) forward fabric-encapsulated packets to network 101 via corresponding border inter-switch ports.

Since border switches 124 and 126 translate between virtual and physical switch identifiers, in the links between fabric switch 102 and compatible network 101, the ingress and/or egress switch identifiers of the encapsulation headers of the packets can be virtual switch identifiers. Hence, compatible network 101 can only view virtual switches 120, 130, and 140 coupled to it instead of fabric switches 102, 103, and 104, respectively. As a result, network 101 can forward traffic only based on the fabric encapsulation without requiring to learn MAC addresses of encapsulated packets. In this way, fabric switches 102, 103, and 104 are transparently interconnected via compatible network 101.

Data Communication

FIG. 2A illustrates an exemplary forwarding of a packet with an unknown destination between transparently interconnected fabric switches, in accordance with an embodiment of the present invention. During operation, end device 162 sends an Ethernet frame 202 to end device 164. Suppose that the destination address of Ethernet frame 202 (i.e., the MAC address of end device 164) is unknown to fabric switch 104. Edge switch 144 receives the packet via an edge port. Switch 144 learns the MAC address of end device 162 and adds the MAC address to its local MAC address table (can also be referred to as forwarding table) in association with the edge port (e.g., based on a port identifier). Switch 144 also generates a notification message comprising the learned MAC address and sends the notification message to switches 142 and 146. In turn, switches 142 and 146 learn the MAC address of end device 162 and add the MAC address to their respective local MAC address tables in association with switch identifier 204 (e.g., an RBridge identifier or an IP address) of switch 144. In some embodiments, switches 142 and 146 further associate the MAC address of end device 162 with the edge port of switch 144 (e.g., based on a port identifier).

Switch 144 encapsulates Ethernet frame 202 with a fabric encapsulation (e.g., TRILL or IP encapsulation) to create fabric-encapsulated packet 222 (operation 232). Since switch 144 does not know the destination (i.e., has not learned the destination MAC address), switch 144 assigns an “all-switch” switch identifier 206 as the egress switch identifier and switch identifier 204 of switch 144 as the ingress switch identifier of the encapsulation header. Switch 144 forwards packet 222 to a respective switch in fabric switch 104. It should be noted that forwarding includes determining an egress (or output) port associated with the destination address and transmitting via the determined egress port.

When packet 222 reaches border switch 142, switch 142 modifies the encapsulation header by changing the ingress switch identifier of the encapsulation header from switch identifier 204 of switch 144 to a virtual switch identifier 206 (e.g., a virtual RBridge identifier or a virtual IP address) of virtual switch 140. Switch 142 forwards packet 222 to switch 116 of network 101 (not shown in FIG. 2A). In this way, switch 142 operates as an egress border switch. Switch 116 forwards packet 222 to a respective switch of network 101. In some embodiments, upon receiving packet 222, a respective switch in network 101 decapsulates packet 222 to extract frame 202 and forwards packet 222 to the virtual switches it is coupled to, as described in conjunction with FIG. 1.

Since virtual switch 120 is reachable via both switches 112 and 114, switches in compatible network 101 can use equal cost multiple path (ECMP) to determine via which switch packet 222 should be forwarded. Suppose that, switch 112 receives and forwards packet 222 to virtual switch 120. In turn, switch 124 of fabric switch 102 receives packet 222 forwarded by switch 112. In this way, border switch 142 of fabric switch 104 transparently forwards fabric-encapsulated packet 222 to border switch 124 of fabric switch 102 via compatible network 101.

Upon receiving packet 222, switch 124 determines that the egress switch identifier is an “all switch” switch identifier and forwards packet 222 to a respective switch in fabric switch 102. In this way, switch 124 operates as an ingress border switch. Switch 124 can also decapsulate packet 222 to obtain the inner packet (i.e., Ethernet frame 202) and determine whether any local end device corresponds to the destination MAC address of Ethernet frame 202. Switch 142 also learns the MAC address of end device 162 and stores the learned MAC address in association with the virtual switch identifier 208 of virtual switch 140. Switch 122 receives packet 222, decapsulates packet 222 to obtain Ethernet frame 202 (operation 236), and determines whether any local end device corresponds to the destination MAC address of Ethernet frame 202. Switch 122 learns the MAC address of end device 162 and stores the learned MAC address in association with the virtual switch identifier 208 of virtual switch 140. Switch 122 also determines that destination end device 164 is locally coupled (e.g., either based on a populated table, previous MAC address learning, or flooding), and forwards Ethernet frame 202 to end device 164.

FIG. 2B illustrates an exemplary forwarding of a packet with a known destination between transparently interconnected fabric switches, in accordance with an embodiment of the present invention. During operation, end device 164 sends an Ethernet frame 212 to end device 162. Edge switch 122 receives the packet via an edge port and encapsulates the packet with a fabric encapsulation (e.g., TRILL or IP encapsulation) to create fabric-encapsulated packet 224 (operation 242). As described in conjunction with FIG. 2A, upon receiving fabric-encapsulated packet 222, switch 122 learns the MAC address of end device 162. Hence, the destination MAC address of Ethernet frame 212 is known to switch 122.

Since switch 122 has associated the MAC address of end device 162 with virtual switch identifier 208 of virtual switch 140, switch 122 assigns virtual switch identifier 208 as the egress switch identifier and a switch identifier 214 (e.g., an RBridge identifier or an IP address) of switch 122 as the ingress switch identifier of the encapsulation header. Switch 122 determines that virtual switch 140 is reachable via switch 124 and forwards packet 224 to switch 124. Route to a respective physical or virtual switch can be configured by a user (e.g., a network administrator) or computed based on a routing protocol (e.g., Intermediate System to Intermediate System (IS-IS)). In some embodiments, the configured or computed routes in fabric switch 102 indicate that virtual switch 140 is reachable via switch 124 and 126.

When packet 224 reaches border switch 124, switch 124 modifies the encapsulation header by changing the ingress switch identifier of the encapsulation header from switch identifier 214 of switch 122 to a virtual switch identifier 216 (e.g., a virtual RBridge identifier or a virtual IP address) of virtual switch 120. Switch 124 forwards packet 224 to switch 112 of network 101 (not shown in FIG. 2B). In this way, switch 124 operates as an egress border switch. Switch 112 determines that virtual switch 140 is reachable via switch 116 and forwards packet 224 to switch 116. Switch 112 forwards the packet in network 101 based on the encapsulation header of packet 224 without decapsulating the fabric encapsulation and learning the MAC address from Ethernet frame 212. Switch 116 receives and forwards packet 224 to virtual switch 140. In turn, border switch 142 of fabric switch 104 receives packet 224 forwarded by switch 116. In this way, border switch 124 of fabric switch 102 transparently forwards fabric-encapsulated packet 224 to border switch 142 of fabric switch 104 via compatible network 101.

Upon receiving the fabric-encapsulated packet, switch 142 determines that the egress switch identifier is virtual switch identifier 208 of virtual switch 140. Switch 142 decapsulate packet 224 to obtain the inner packet (i.e., Ethernet frame 212) and identifies the switch with which the destination MAC address of Ethernet frame 212 (i.e., the MAC address of end device 162) is associated. Switch 142 has associated the MAC address of end device 162 with switch identifier 204 of switch 144, as described in conjunction with FIG. 2A. Switch 142 modifies the encapsulation header by changing the egress switch identifier of the encapsulation header from virtual switch identifier 208 of virtual switch 140 to switch identifier 204 of switch 144 (operation 246). In this way, switch 142 operates as an ingress border switch. In some embodiments, switch 142 re-encapsulates Ethernet frame 212 in an encapsulation header, and assigns the switch identifier 204 as the egress switch identifier and virtual switch identifier 216 as the ingress switch identifier of the encapsulation header.

Switch 142 also learns the MAC address of end device 164 and stores the learned MAC address in association with the virtual switch identifier 216 of virtual switch 120. Switch 142 forwards packet 224 to switch 144. Switch 144 receives packet 224, decapsulates packet 224 to obtain Ethernet frame 212 (operation 248), and determines whether any local end device corresponds to the destination MAC address of Ethernet frame 212. Switch 144 also learns the MAC address of end device 164 and stores the learned MAC address in association with the virtual switch identifier 216 of virtual switch 120. Switch 144 determines that destination end device 162 is locally coupled (e.g., either based on a populated table, previous MAC address learning, or flooding), and forwards Ethernet frame 212 to end device 162.

Forwarding of a Packet with Unknown Destination

In the example in FIG. 2A, edge switch 144 receives a packet (i.e., Ethernet frame 202) with an unknown destination from a local end device 162. Switch 144 encapsulates this packet in a fabric encapsulation and forwards the fabric-encapsulated packet in fabric switch 104. Egress border switch 142 receives the fabric-encapsulated packet and forwards via a border inter-switch port to switch 116. This fabric-encapsulated packet is forwarded in network 101 and reaches ingress border switch 124 of fabric switch 102, which, in turn, forwards the fabric-encapsulated packet in fabric switch 102.

FIG. 3A presents a flowchart illustrating the process of an edge switch forwarding a packet with an unknown destination received from a local end device, in accordance with an embodiment of the present invention. During operation, the switch receives a packet with an unknown destination from a local device via an edge port (operation 302). The switch encapsulates the packet with fabric encapsulation and assign an “all switch” switch identifier as the egress switch identifier of the encapsulation header (operation 304). For example, if the fabric encapsulation is based on the TRILL protocol, the “all switch” switch identifier can be a multicast RBridge identifier. The switch sets the local switch identifier as the ingress switch identifier of the encapsulation header (operation 306) and sends the fabric-encapsulated packet based on the fabric “all switch” forwarding policy (operation 308). Examples of a fabric “all switch” forwarding policy include, but are not limited to, forwarding via fabric multicast tree, forwarding via a multicast tree rooted at an egress switch, unicast forwarding to a respective member of the fabric switch, and broadcast forwarding in the fabric switch.

FIG. 3B presents a flowchart illustrating the process of an egress border switch forwarding a packet with an unknown destination, in accordance with an embodiment of the present invention. During operation, the switch receives a fabric-encapsulated packet with “all switch” switch identifier as the egress switch identifier of the encapsulation header via an inter-switch port (operation 332). The switch then identifies the ingress switch identifier in the encapsulation header in (a copy of) the fabric-encapsulated packet (operation 334) and modifies the encapsulation header by replacing the identified switch identifier with a virtual switch identifier of a local virtual switch (operation 336). This local virtual switch represents the local fabric switch in which the switch is a member switch. The switch then identifies a local border inter-switch port associated with (e.g., mapped to) the egress switch identifier of the encapsulation header as an egress port for the packet (operation 338) and forwards the packet via the identified port (operation 340).

In some embodiments, the switch also decapsulates (a copy of) the fabric-encapsulated packet to extract the inner packet (operation 342). This allows the switch to determine whether any local end device is the destination of the inner packet. The switch then checks whether the destination MAC address of the inner packet has been locally learned (e.g., the switch has learned the destination MAC address from a local edge port) (operation 344). If the packet has been locally learned, the switch identifies an egress edge port associated with the destination MAC address of the inner packet and forwards the inner packet via the identified port (operation 346). Otherwise, the switch forwards the inner packet via a respective local edge port (operation 348).

FIG. 3C presents a flowchart illustrating the process of an ingress border switch forwarding a packet with an unknown destination, in accordance with an embodiment of the present invention. During operation, the switch receives a fabric-encapsulated packet with “all switch” switch identifier as the egress switch identifier of the encapsulation header via a border inter-switch port (operation 352). The switch then identifies an “all switch” switch identifier as the egress switch identifier in the encapsulation header in the fabric-encapsulated packet (operation 354) and sends the fabric-encapsulated packet based on the fabric “all switch” forwarding policy (operation 356).

In some embodiments, the switch also decapsulates (a copy of) the fabric-encapsulated packet to extract the inner packet (operation 358). This allows the switch to determine whether any local end device is the destination of the inner packet. The switch then checks whether the destination MAC address of the inner packet has been locally learned (e.g., the switch has learned the destination MAC address from a local edge port) (operation 360). If the packet has been locally learned, the switch identifies an egress edge port corresponding to the destination MAC address of the inner packet and forwards the inner packet via the identified port (operation 362). Otherwise, the switch forwards the inner packet via a respective local edge port (operation 364).

Forwarding of a Packet with Known Destination

In the example in FIG. 2B, edge switch 122 receives a packet (i.e., Ethernet frame 212) with a known destination from a local end device 164. Switch 122 encapsulates this packet in a fabric encapsulation and forwards the fabric-encapsulated packet in fabric switch 102. Egress border switch 124 receives the fabric-encapsulated packet and forwards via a border inter-switch port to switch 112. This fabric-encapsulated packet is forwarded in network 101 and reaches ingress border switch 142 of fabric switch 104, which, in turn, forwards the fabric-encapsulated packet in fabric switch 104.

FIG. 4A presents a flowchart illustrating the process of an edge switch forwarding a packet with a known destination received from a local end device, in accordance with an embodiment of the present invention. During operation, the switch receives a packet (e.g., an Ethernet frame) from a local device via an edge port (operation 402). The switch obtains the switch identifier mapped to the destination MAC address of the packet from a local MAC table (operation 404). The switch encapsulates the packet with a fabric encapsulation (operation 406). It should be noted that this switch identifier can be a virtual switch identifier if the destination is behind a border inter-switch port.

The switch sets the local switch identifier as the ingress switch identifier and the identified switch identifier as the egress switch identifier of the encapsulation header (operation 408). The switch identifies the next-hop switch identifier(s) mapped to the identified switch identifier (e.g., from a local forwarding table) and selects a switch identifier from the identified next-hop switch identifier(s) (operation 410). The switch identifies an egress port corresponding to the selected next-hop switch identifier (e.g., from a local forwarding table) and forwards the encapsulated packet via the identified port (operation 412).

FIG. 4B presents a flowchart illustrating the process of an egress border switch forwarding a packet with a known destination, in accordance with an embodiment of the present invention. During operation, the switch receives a fabric-encapsulated packet via an inter-switch port (operation 432). The switch then identifies the egress switch identifier in the encapsulation header in the fabric-encapsulated packet (operation 434) and checks whether the identified switch identifier is an external switch identifier (e.g., not associated with a switch in the local fabric switch) (operation 436). It should be mentioned that a respective member switch in a fabric switch stores the switch identifier of a respective member switch of the local fabric switch. The switch modifies the encapsulation header by changing the ingress switch identifier of the encapsulation header in with a virtual switch identifier of a local virtual switch (operation 438). The switch then identifies a local border inter-switch port associated with the egress switch identifier of the encapsulation header as an egress port for the packet (operation 440) and forwards the packet via the identified port (operation 442).

FIG. 4C presents a flowchart illustrating the process of an ingress border switch forwarding a packet with a known destination, in accordance with an embodiment of the present invention. During operation, the switch receives a fabric-encapsulated packet via a border inter-switch port (operation 452). The switch then identifies the egress switch identifier in the encapsulation header of the fabric-encapsulated packet (operation 354) and checks whether the egress switch identifier is a local virtual switch identifier (operation 456). A local virtual switch identifier is associated with the virtual switch represented by the local fabric switch (i.e., the fabric switch in which the switch is a member).

If the egress switch identifier is a local virtual switch identifier, the switch modifies the encapsulation header by changing the identified switch identifier with a switch identifier associated with the destination MAC address of the inner packet of the fabric-encapsulated packet (operation 460). In some embodiments, the switch decapsulates the fabric-encapsulated packet and re-encapsulates the inner packet to perform operation 460. The switch identifies an egress port associated with the egress switch identifier (operation 462) and forwards the encapsulated packet via the identified port (operation 464).

Exemplary Switch

FIG. 5 illustrates an exemplary switch with transparent fabric switch interconnection support, in accordance with an embodiment of the present invention. In this example, a switch 500 includes a number of communication ports 502, a packet processor 510, a border module 520, and a storage device 550. Packet processor 510 extracts and processes header information from the received frames.

In some embodiments, switch 500 maintains a membership in a fabric switch, as described in conjunction with FIG. 1, wherein switch 500 also includes a fabric switch module 560. Fabric switch module 560 maintains a configuration database in storage device 550 that maintains the configuration state of every switch within the fabric switch. Fabric switch module 560 maintains the state of the fabric switch, which is used to join other switches. In some embodiments, switch 500 can be configured to operate in conjunction with a remote switch as an Ethernet switch.

Communication ports 502 can include inter-switch communication channels for communication within the fabric switch. This inter-switch communication channel can be implemented via a regular communication port and based on any open or proprietary format. Communication ports 502 can also include one or more border inter-switch communication ports for communication via compatible networks. Communication ports 502 can include one or more TRILL ports capable of receiving frames encapsulated in a TRILL header. Communication ports 502 can also include one or more IP ports capable of receiving IP packets. An IP port is capable of receiving an IP packet and can be configured with an IP address. Packet processor 510 can process TRILL-encapsulated frames and/or IP packets.

During operation, border module 520 determines that an egress switch identifier in the encapsulation header of a packet is associated with a switch outside of the fabric switch. In response to the determination, border module 520 changes the ingress switch identifier in the encapsulation header to a virtual switch identifier associated with a virtual switch representing the fabric switch. On the other hand, if border module 520 determines that an egress switch identifier in an encapsulation header of a packet is the virtual switch identifier, border module 520 changes the egress switch identifier in the encapsulation header to a switch identifier which identifies a member switch in the fabric switch.

In some embodiments, switch 500 includes a forwarding module 530 which determines that an egress switch identifier in the encapsulation header of a packet is a switch identifier of the switch. The ingress switch identifier in the encapsulation header can be a virtual switch identifier associated with a remote fabric switch. Forwarding module 530 can also determine an external switch as a next-hop switch in a compatible network for a fabric-encapsulated packet. Switch 500 can further include a learning module 540 which learns the MAC address from the inner packet of the packet and stores the learned MAC address in association with the virtual switch identifier associated with the remote fabric switch in storage device 550.

Note that the above-mentioned modules can be implemented in hardware as well as in software. In one embodiment, these modules can be embodied in computer-executable instructions stored in a memory which is coupled to one or more processors in switch 500. When executed, these instructions cause the processor(s) to perform the aforementioned functions.

In summary, embodiments of the present invention provide a switch and a method for transparently interconnecting fabric switches. In one embodiment, the switch includes a fabric switch module and a border module. The fabric switch module maintains a membership in a first fabric switch. A fabric switch includes a plurality of switches and operates as a single switch. The border module determines that the egress switch identifier in a first encapsulation header of a first packet is associated with a switch outside of the fabric switch. The first packet is forwarded in the first fabric switch based on the first encapsulation header. In response to the determination, the border module changes the ingress switch identifier in the first encapsulation header of the first packet to a first virtual switch identifier associated with a first virtual switch. This first virtual switch externally represents the first fabric switch.

The methods and processes described herein can be embodied as code and/or data, which can be stored in a computer-readable non-transitory storage medium. When a computer system reads and executes the code and/or data stored on the computer-readable non-transitory storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the medium.

The methods and processes described herein can be executed by and/or included in hardware modules or apparatus. These modules or apparatus may include, but are not limited to, an application-specific integrated circuit (ASIC) chip, a field-programmable gate array (FPGA), a dedicated or shared processor that executes a particular software module or a piece of code at a particular time, and/or other programmable-logic devices now known or later developed. When the hardware modules or apparatus are activated, they perform the methods and processes included within them.

The foregoing descriptions of embodiments of the present invention have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit this disclosure. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. The scope of the present invention is defined by the appended claims. 

What is claimed is:
 1. A switch, comprising: a fabric switch module adapted to maintain a membership in a first fabric switch, wherein the fabric switch includes a plurality of switches and operates as a single logical switch; a border module adapted to: determine that an egress switch identifier in a first encapsulation header of a first packet is associated with a switch outside of the fabric switch, wherein the first packet is forwardable in the first fabric switch based on the first encapsulation header; and in response to the determination, change an ingress switch identifier in the first encapsulation header of the first packet to a first virtual switch identifier associated with a first virtual switch, wherein the first virtual switch externally represents the first fabric switch, thereby facilitating interconnection of fabric switches.
 2. The switch of claim 1, wherein the egress switch identifier in the first encapsulation header is a second virtual switch identifier associated with a second virtual switch, wherein the second virtual switch externally represents a second fabric switch.
 3. The switch of claim 2, wherein routing information of the first fabric switch indicates that the second virtual switch is reachable via the switch.
 4. The switch of claim 2, wherein the border module is further adapted to: determine that an egress switch identifier in a second encapsulation header of a second packet is the first virtual switch identifier; and in response to the determination, change the egress switch identifier in the second encapsulation header of the second packet to a switch identifier which identifies a member switch in the first fabric switch.
 5. The switch of claim 2, wherein an ingress switch identifier in the second encapsulation header of the second packet is the second virtual switch identifier.
 6. The switch of claim 2, further comprising: a forwarding module adapted to determine that an egress switch identifier in a third encapsulation header of a third packet is a switch identifier of the switch, wherein an ingress switch identifier in the second encapsulation header of the third packet is the second virtual switch identifier; and a learning module adapted to: learn a media access control (MAC) address from an inner packet of the third packet; and store the learned MAC address in association with the second virtual switch identifier in a storage device.
 7. The switch of claim 1, further comprising a forwarding module adapted to determine an external switch as a next-hop switch for the first packet based on the first encapsulation header, wherein the external switch is not a member switch of the first fabric switch.
 8. The switch of claim 1, wherein the first encapsulation header is one or more of: a Transparent Interconnection of Lots of Links (TRILL) header, wherein the ingress and egress switch identifiers of the first encapsulation header are TRILL routing bridge (RBridge) identifiers; and a Internet Protocol (IP) header, wherein the ingress and egress switch identifiers of the first encapsulation header are IP addresses.
 9. A method, comprising: maintaining a membership of a switch in a first fabric switch, wherein the fabric switch includes a plurality of switches and operates as a single logical switch; determining that an egress switch identifier in a first encapsulation header of a first packet is associated with a switch outside of the fabric switch, wherein the first packet is forwardable in the first fabric switch based on the first encapsulation header; and in response to the determination, changing an ingress switch identifier in the first encapsulation header of the first packet to a first virtual switch identifier associated with a first virtual switch, wherein the first virtual switch externally represents the first fabric switch, thereby facilitating interconnection of fabric switches.
 10. The method of claim 9, wherein the egress switch identifier in the first encapsulation header is a second virtual switch identifier associated with a second virtual switch, wherein the second virtual switch externally represents a second fabric switch.
 11. The method of claim 10, wherein routing information of the first fabric switch indicates that the second virtual switch is reachable via the switch.
 12. The method of claim 10, further comprising: determining that an egress switch identifier in a second encapsulation header of a second packet is the first virtual switch identifier; and in response to the determination, changing the egress switch identifier in the second encapsulation header of the second packet to a switch identifier which identifies a member switch in the first fabric switch.
 13. The method of claim 10, wherein an ingress switch identifier in the second encapsulation header of the second packet is the second virtual switch identifier.
 14. The method of claim 10, further comprising: determining that an egress switch identifier in a third encapsulation header of a third packet is a switch identifier of the switch, wherein an ingress switch identifier in the second encapsulation header of the third packet is the second virtual switch identifier; learning a media access control (MAC) address from an inner packet of the third packet; and storing the learned MAC address in association with the second virtual switch identifier in a storage device.
 15. The method of claim 9, further comprising determining an external switch as a next-hop switch for the first packet based on the first encapsulation header, wherein the external switch is not a member switch of the first fabric switch.
 16. The method of claim 9, wherein the first encapsulation header is one or more of: a Transparent Interconnection of Lots of Links (TRILL) header, wherein the ingress and egress switch identifiers of the first encapsulation header are TRILL routing bridge (RBridge) identifiers; and a Internet Protocol (IP) header, wherein the ingress and egress switch identifiers of the first encapsulation header are IP addresses.
 17. A computer system; comprising: a processor; a storage device coupled to the processor and storing instructions that when executed by the processor cause the processor to perform a method, the method comprising: maintaining a membership of a switch in a first fabric switch, wherein the fabric switch includes a plurality of switches and operates as a single logical switch; determining that an egress switch identifier in a first encapsulation header of a first packet is associated with a switch outside of the fabric switch, wherein the first packet is forwardable in the first fabric switch based on the first encapsulation header; and in response to the determination, changing an ingress switch identifier in the first encapsulation header of the first packet to a first virtual switch identifier associated with a first virtual switch, wherein the first virtual switch externally represents the first fabric switch, thereby facilitating interconnection of fabric switches.
 18. The computer system of claim 17, wherein the egress switch identifier in the first encapsulation header is a second virtual switch identifier associated with a second virtual switch, wherein the second virtual switch externally represents a second fabric switch.
 19. The computer system of claim 18, wherein routing information of the first fabric switch indicates that the second virtual switch is reachable via the switch.
 20. The computer system of claim 18, wherein the method further comprises: determining that an egress switch identifier in a second encapsulation header of a second packet is the first virtual switch identifier; and in response to the determination, changing the egress switch identifier in the second encapsulation header of the second packet to a switch identifier which identifies a member switch in the first fabric switch.
 21. The computer system of claim 18, wherein an ingress switch identifier in the second encapsulation header of the second packet is the second virtual switch identifier.
 22. The computer system of claim 18, wherein the method further comprises: determining that an egress switch identifier in a third encapsulation header of a third packet is a switch identifier of the switch, wherein an ingress switch identifier in the second encapsulation header of the third packet is the second virtual switch identifier; learning a media access control (MAC) address from an inner packet of the third packet; and storing the learned MAC address in association with the second virtual switch identifier in a storage device.
 23. The computer system of claim 17, wherein the method further comprises determining an external switch as a next-hop switch for the first packet based on the first encapsulation header, wherein the external switch is not a member switch of the first fabric switch.
 24. The computer system of claim 17, wherein the first encapsulation header is one or more of: a Transparent Interconnection of Lots of Links (TRILL) header, wherein the ingress and egress switch identifiers of the first encapsulation header are TRILL routing bridge (RBridge) identifiers; and a Internet Protocol (IP) header, wherein the ingress and egress switch identifiers of the first encapsulation header are IP addresses. 